Dryer control system



June 27, 1967 HOOD DRYER CONTROL SYSTEM Filed 001;. 22, 1965 INVENTOR ROBERT B, HOOD AGENT United States Patent 3,327,403 DRYER CONTRGL SYSTEM Robert B. Hood, Logansport, Ind, assignor to Essex Wire Corporation, Fort Wayne, laid, a corporation of Michigan Filed (lot. 22, 1965, Ser. No. 591,655 9 Claims. (Cl. 34-45) This invention relates to domestic clothes drying machines and more particularly to an electrical control system used in such machines.

It is known to control the operation of a clothes drying machine by systems which sense the actual moisture condition of the clothes. Such systems generally have electrodes engaged by the clothing to sense the electrical resistance which is a function of the dryness. The electrodes partially discharge the capacitor of a R-C network whenever a sufficiently moist clothing area is sensed. After the clothing has dried sufficiently, the capacitor charges at a faster rate than it can be discharged by the electrodes. When the capacitor charge reaches a pre-determined value, it operates a switch which terminates the dryer operation. As certain types and weights of clothes may give a false indication of dryness, the above described control system is not always effective when completely dried clothes are desired. To overcome this problem, it has been proposed to continue operation of the dryer following the first sensing of an apparently dry condition of clothes and obtained a repeated sensing of a dry condition before the dryer operation is terminated. A control system for such operation has the disadvantage of requiring the use of a somewhat expensive switching device. Furthermore, such a system cannot provide the necessary additional sensing cycles when an extremely damp portion of clothes is present in the second sensing period.

Accordingly, it is an object of this invention to provide an improved dryer control system operative to sense the electrical resistance of the clothes being dried and to maintain the dryer in operation until all parts of the clothes have been dried.

Another object of this invention is to provide a dryer control system having a moisture sensing circuit which has an R-C timing network and uses relatively low cost components.

A further object of this invention is to provide a new and a highly advantageous moisture sensing circuit for a dryer control system which comprises two rectifier circuits connected to convert an alternating current voltage into two unidirectional voltages of opposite polarity for the charging of a capacitor in a RC timing circuit.

Other objects and advantages of the invention will become apparent with reference to the following description of the preferred embodiment of the invention shown in the accompanied drawing in which:

The sole figure is an electrical schematic diagram of a circuit illustrating a dryer control system incorporating the novel features of the present invention.

Referring to the drawing, there is shown a dryer control system for use with domestic clothes drying machine of a well known type in which a drum or basket containing clothes to be dried is rotatably driven by an electric motor 10. The motor It) also drives a blower to circulate heated air through the dryer drum as the clothes are tumbled. The circulating air is heated by a heating unit which may be an electric resistance heater 11 as illustrated in the figure or may be a gas burner supplied with gas under the control of the usual electromagnetic valve. Where the heating unit is a gas burner, the solenoid coil of the main gas valve is controlled by the control system of this invention in the same manner as is 3,327,403 Patented June 27, 1967 ice the heater 11 in an electrically heated dryer. The dryer control system is energized from the usual 3-wire alternating current supply circuit by the supply lines 12, 13, and 14. The lines 12 and 14 will be normally connected to a 240 volt power supply system with 120 volts ap pearing between each of these lines and the neutral line 13. The motor 10 includes a main winding 15 and a startlng winding 16 both connected at a common end to a conductor 17. The conductor 17 is connected to the supply line 13 by the contacts 18a of a conventional door switch 18 which is closed when the dryer door is closed and opened when the dryer door is opened to prevent operation of the motor 10 with the door opened. The other ends of the motor windings 15 and 16 are connected in parallel by the contact arm 19 and the contact 26 of a speed-responsive switch 21. The speed-responsive switch 21 and a second speed-responsive switch 22 may be controlled by a centrifugal mechanism or the like which moves the contact arm 19 from engagement with the contact 29 into engagement with the contact 23 and opens the switch 22 when the motor has accelerated to a predetermined speed. The contact 23 is connected by a thermostatic cool-down switch 24 to the line 12. The thermostatic switch 24 which is responsive to the temperature of the air heated by the heater 11 closes when the air temperature rises above a predetermined value and reopens at a temperature below that value. The switch 24 thus provides for continued operation of the motor 10 for a limited period of time after the heater 11 is deenergized. The contact arm 19 of the speed-responsive switch 21 at its junction with the motor winding 15 is connected with the line 12 by the normally open switch 25 of the power relay 26.

The relay 26 also includes a second normally open switch 27, a third normally open switch 23, and a coil 29. The relay 26 which is of conventional construction closes its normally open switches 25, 27 and 28 when the coil 29 is energized. One side of the relay coil 29 is connected to the line 12 by the relay switch 28 and the other side is connected to the conductor 17 by the normally closed switch 39 of the relay 31. The relay 31 is also of conventionalL construction and opens the switch when its coil 32 is energized in a manner hereinafter described.

The line 12 is connected by the relay switch 27 and the contacts 34 of a selector switch 35 to one end of the heater 11. The selector switch 35 which also includes a second set of contacts 36 permits the selection of two different types of dryer operation. With the contacts 34 and 346 closed, heated air is provided for drying of the clothes in the dryer. When the contacts 34 and 36 are opened, the clothes are tumbled in unheated air. The other end of the heater 11 is connected in series with a conventional safety thermostat 37 and the motor speed responsive switch 22. The thermostat 37 is located in the heated air flow from the heater 11 to deenergize the heater 11 in case the dryer temperature exceeds a safe value. The speed responsive switch 22 prevents energization of the heater 11 in case the motor is fails to operate. To initiate operation of the dryer, there is provided a manually operated momentary start switch 38 having two pairs of normally open contacts 39 and 40. The contacts as are connected in parallel with the relay switch 28 between the line 12 and the junction 33 of the relay coil 29 with the relay switch 28.

The dryer control system of this invention includes a novel moisture sensing and time delay circuit 41 energized from the supply circuit by means of a step-up transformer 42. The primary winding 43 of the transformer 42 is connected to the junction 33 and the conductor 17. The transformer 42 has a secondary winding 44 with two terminal leads 45 and 46 supplying an alternating current voltage of approximately 260 volts. Two rectifying circuits are connected between the lead 45 and a junction 47. The first rectifying circuit includes a rectifier 48 in series connection with a resistor 49 and a pair of electrodes t) and 51 which are mounted within a dryer drum rotated by the motor 19. The electrode 5t) and 51 may comprise suitably spaced wires exposed to the clothes contained in the dryer drum such as is described in Great Britain Patent No. 877,553 issued September 13, 1961, to the Maytag Company. Damp clothes coming into engagement with the electrodes 50 and 51 electrically bridge or contact the electrodes. Connected in parallel with the resistor 49 and the electrodes 54 and 51 is a branch circuit consisting of a resistor 52 in series connection with the contacts 49* of the start switch 38.

The second rectifying circuit includes a second rectifier 53 in series connection with a resistor 54. The resistor 54 is preferably of the potentiometer type having a movable contact 55 selectively engageable with any one of the tap points 54a, 54b, 54c, and 54d of the resistor. The contact 55 is connected to the junction 47 by the contacts 36 of the selector switch 35. When the contacts 36 are closed, the resistance of the second rectifying circuit is varied in predetermined steps by movement of the contact 55. The resistor 54 has an effective resistance value at least several times as great as the resistance values of the resistors 43 and 52.

Connected between the transformer lead 45 and the junction 47 is a capacitor 56 which is charged from the two rectifying circuits described. It can be seen that the two rectifiers 48 and 53 convert the alternating voltage appearing on the transformer leads 45 and 46 into unidirectional voltages of opposite polarities across the capacitor 56. Connected in parallel with the capacitor 56 is a resistor 57 and a gaseous discharge device 58 such as a neon glow tube. A branch circuit consisting of a rectifier 59 in series with the coil 32 of the relay 31 is connected across the resistor 57 and in series with the discharge device 58. The door switch 18 has a second pair of contacts 18b connected in series with a discharge resistor 60 across the capacitor 56 so that the latter is discharged and cannot be recharged whenever the dryer door is opened.

The discharge device 58 has a definite break-down potential at which it becomes conductive. There may be some differences in the break-down potential for opposite polarities. In the following, the term forward firing voltage refers to the breakdown potential of the discharge device 58 which is positive with respect to the lead 45 and the term reverse firing voltage refers to the breakdown potential of opposite polarity. As soon as the discharge device58 becomes conductive, the voltage across it drops somewhat to the so called maintaining voltage which is substantially constant and independent of the discharging current. When the voltage across the discharge device 58 is reduced to the lower limit of the maintaining voltage, the current discharge to the discharge device 58 is abruptly discontinued.

The operation of the dryer control system shown in the figure will now be described. To operate the dryer through an automatic drying cycle, the selector switch 35 is set to its closed condition shown in the figure. Initially, the system is in the normal or deenergized condition in which the coils of the relays 26 and 31 are deenergized and their switches are in the positions in which they are illustrated. The capacitor 55 is discharged as it is shunted by the resistor 60 through the closed door switch 1812 when the dryer door is opened. When wet or damp clothes are placed in the dryer drum and the start switch 38 is momentarily operated to initiative dryer operation, the coil 29 of relay 26 is initially energized in a circuit extending from the supply line 12 through the start switch contacts 39, junction 33, coil 29, relay switch 38, conductor 17, and door switch contacts 18a to the supply line 13.

The energization of the relay coil 29 causes the relay switches 25, 27 and 28 to close. The relay switch 25 completes a circuit to the main winding 15 of the motor 18 extending from the supply line 12 through the relay switch 25, main winding 15, conductor 17, and the door switch contacts 18a to the supply line 18. As the motor starting winding 16 is connected in parallel with the main winding 15 by the contact arm 19 and the contact 20, the motor 10 starts. As the motor 10 approaches its normal operating speed, the contact arm 19 separates from the contact 20 and engages the contact 23 thereby disconnecting the starting winding 16 and connecting the cool-down switch 24 in parallel with the relay switch 25. At the same time, the motor switch 22 closes. The closing of the relay switch 28 establishes aholding circuit for the relay coil 29 across the start switch contacts 39. The closing of the relay switch 27 and the subsequent closing of the motor speedresponsive switch 22 complete a circuit to the heater 11 extending from the supply line 12 through relay switch 27, selector switch contacts 34, heater 11, thermostat 37, and motor switch 22 to the supply line 14.

The closing of the relay switch 28 also energizes the primary winding 43 of the transformer 42 causing an alternating current voltage to be impressed upon the leads 45 and 46. This voltage tends to effect charging of the capacitor 5521s the discharge circuit through the resistor 60 is interrupted by the opening of the door switch contacts 18b when the dryer door is closed. While the start switch 38 is held in its operated position, the rectifier 48 converts the alternating current voltage to a unidirectional voltage tending to charge the capacitor to a positive potential with respect to the lead 45. This charging circuit extends from lead 46 through the closed start switch contacts 40, resistor 52, rectifier 48, junction 47 and capacitor 56 to the lead 45. At the same time, the rectifier 53 converts the alternating voltage to a unidirectional voltage tending to charge the capacitor 56 to a negative potential in a charging circuit extending from lead 46 through rectifier 53, the eifective portion of resistor 54, selector switch contacts 36, junction 47 and the capacitor 56 to the lead 45. As the effective resistance of the resistor 54 is several times as great as the resistance of resistor 52, the charging current through rectifier 48 is much greater than that through the rectifier 53. The capacitor 56 will therefore quickly charge with a positive potential to the forward firing voltage of the discharge device 58 and causes break-down or firing of the discharge device 58. The voltage across the capacitor 56 then drops to a value equal to the sum of the maintaining voltage of the discharge device 58 and the voltage drop across resistor 57 as current flows through the latter and the discharge device 58. There is, however, no current flow through the relay coil 32 at this time as such current is blocked by the rectifier 59 when the capacitor 56 has a positive potential.

The charging circuit through the resistor 52 is then interrupted when the start switch 38 is released and its contacts 48 open. If there are wet or damp clothes in the dryer drum, they will be tumbled against the electrodes 50 and 51. The moisture in the clothes provides a conductive path across the electrodes which permits current to flow in a charging circuit extending from the lead 4-6 through electrodes 56 and 51, resistor 49, rectifier 48, junction 47, and the capacitor 56 to the lead 45. Under these conditions the voltage across the capacitor 56 remains at a voltage equal to the sum of maintaining voltage of the break-down device 58 and the voltage drop across the resistor 57.

As the clothes dry during continued dryer operation, the electrical resistance of the clothes engaging the electrodes 5t) and 51 increases considerably to reduce the current flow through the rectifier 48. As the capacitor charging effect of the first rectifier circuit decreases, thepositive voltage across the capacitor 56 will drop below the maintaining voltage of the discharge device 58 and eventually reverse in polarity. It will be understood that the rectifiers 48 and 53 are conducting on alternate half cycles of the voltage across the leads 45 and 46 and that at this time the voltage component contributed by the rectifier 53 to the resultant voltage across the capacitor 56 has become greater than the component of opposite polarity contributed by the rectifier 48. With further drying of the clothes, the current flow across the electrodes 50 and 51 is further reduced, permitting the capacitor 56 to charge to a negative potential with respect to the lead 45 at a rate determined primarily by the capacitance value of the capacitor 56 and the effective resistance value of the resistance 54. The negative potential across the capacitor gradually increases until it reaches the reverse firing voltage of the discharge device 58 causing the latter to become conductive. The capacitor 56 then discharges through the discharge device 58, the rectifier 59 in its forward direction and the coil 32 of the relay 31, thereby opening the relay switch 39.

The opening of the relay switch 39 deenergizes the coil 29 of the relay 26 causing the relay switches 25, 27 and 23 to reopen. The opening of the relay switch 28 interrupts the holding circuit of the relay coil 29 and the opening of the relay switch 27 deenergizes the heater 11. The opening of the relay switch 25 interrupts the initial energizing circuit for the motor but the motor 10 remains energized through the shunt circuit provided by the cool-down switch 24, the contact 23 and the contact arm 19 of the motor switch 21. The cool-down switch 24 was previously actuated to its closed condition by heated air within the dryer and permits continued operation of the motor 10 in order to circulate unheated air through the dryer drum. After a short time, the cool-down switch 24 cools and opens to terminate the dryer operation.

It will be observed that in the above described operation of the control system, the automatic drying cycle comprises an initial untimed drying period and a final timed drying period. The initial untimed period begins with the initial charging of the capacitor 56 to a positive value through the resistor 52 and the start switch contact 49. As long as damp clothes engage the electrodes 59 and 51 to maintain the capacitor 56 charged to a positive volta e, the untimed period continues. However, as the clothes approach a dried condition and their resistance increases to a correspondingly higher value, the current flow through the resistor 49 and the rectifier 48 is substantially reduced. As a result, the current fiow through the rectifier 53 and the resistor 54 becomes efiective to decrease and then reverse the voltage across the capacitor 55, thus starting the final timed period. The conductivity of the clothes and their corresponding moisture condition at the beginning of the final timed period will be determined by the effective resistance value of the resistor 54. The length of the timed final period is determined by the selected value of the effective resistance of the resistor 54 in the R-C circuit comprising the resistor 54 and the capacitor 56. In this way any desired degree of dryness can be obtained.

In the event, the dryer contains clothes which dry unevenly, the clothes may exhibit an apparently dry condition to the electrodes 50 and 51 before all wet or damp portions of the clothes have dried. Thus the final timed drying period may begin under such conditions. However as soon as a damp or wet portion of the clothes previously trapped within dry portions of the clothes comes into engagement with the electrodes 50 and 51, the resulting current flow through the resistor 49 and the rectifier 48 recharges the capacitor 56 to a positive voltage. This restores the circuit conditions to that of the initial timed period. This reset action prevents termination of the dryer until all parts of the clothes have been dried.

In addition to the foregoing automatic drying cycle, the control system may be set to provide the tumbling of clothes in a stream of unheated air by manually opening the selector switch 35. Actuation of the start switch 38 energizes the relay coil 29 and starts the motor 10 as described above. However, the energizing circuit for the heater 11 is interrupted by the opened contacts 34 of the selector switch 35 and the heater remains inoperative through the cycle. The moisture sensing circuit also will function as described above except the full resistance of the resistor 54 is effective as the opened contacts 36 of the selector switch 35 interrupt the shunt circuit extending from the resistor contact 55 to the junction 47. As no heat is used in this operation, the cool-down switch 24 does not close and consequently the dryer motor 10 stops immediately when the relay 26 opens its switch 25 at the end of the timed final period.

At any time during dryer operation when the door switch contacts 18a are opened by the opening of the dryer door, the energizing circuits of the motor 10, the relay 26, and the transformer 42 are interrupted. The heater 11 is also deenergized as the relay switch 27 opens and the motor switch 22 opens. At the same time, the capacitor 56 is discharged through the resistor 60 and the door switch contacts 18b to prevent the operator of the dryer from receiving an electrical shock if the electrodes 50 and 51 are touched. Thus the operator may terminate operation of the dryer at any time by merely opening the dryer door. If it is desired to resume operation of the dryer, the reclosing of the door switch 18 and the momentary actuation of start switch 38 will cause the dryer control system to repeat its entire operating cycle as described above.

It will be apparent that the control system shown in the figure may be modified by omitting or disconnecting the circuit branch comprising the start switch contacts and the resistor 52. With this modification, the capacitor 56 will be uncharged at the beginning of dryer operation. If the dryer drum contains wet clothes, the bridging of the electrodes and 51 by the clothes initially causes the capacitor 56 to charge to a positive voltage. Thus the modified control system will provide a complete automatic drying cycle as described above. However, if the operator places dried clothes in the dryer drum in order to flufi them out, for instance, the capacitor 56 will immediately begin to charge with a negative voltage. It will be seen that under these conditions the timed final period is one half that required in the control system shown in the figure. In the latter control system, the capacitor 56 does not begin charging with a negative voltage until the voltage across the capacitor 56 drops to zero level half way through the timed final period.

What is claimed is:

1. In a control system for a clothes drying machine having an electric motor for rotating a drum in order to tumble clothes contained therein to be dried, heating means arranged to heat clothes within said drum, and spaced electrodes positioned within said drum and exposed to clothes therein in order to be electrically bridged by wet clothes tumbled in said drum: The combination therewith of:

an alternating current supply circuit;

a pair of terminal leads coupled to said supply circuit for energization with an alternating voltage;

a capacitor;

a first rectifier circuit including in series with said electrodes a first resistor and a first rectifier; said first rectifier circuit being connected in series with said capacitor across said terminal leads to convert said alternating voltage into a unidirectional voltage of a first polarity across said capacitor;

a second rectifier circuit connected in parallel with said first rectifier circuit and including in series connection a second resistor and a second rectifier oppositely poled with respect to said first rectifier to convert said alternating voltage into an unidirectional voltage across said capacitor of a second polarity opposite to that said first unidirectional voltage;

said first resistor having a resistance value less than that of said second resistor so that said first rectifier circuit is effective to charge said capacitor with voltage of said first polarity when said electrodes are bridged by damp clothes tumbled in said drum;

said second rectifier circuit with said capacitor comprising an RC timing circuit effective to discharge and then recharge said capacitor to a predetermined voltage of said second polarity when the electrical resistance of clothes engaging said electrodes is greater than a predetermined value;

electric means effective when energized to connect said heating means to said supply circuit; and

means responsive to the voltage across said capacitor to deenergize said electric means when said capacitor charges to said predetermined voltage of said second polarity.

2. The combination according to claim 1 in which said means responsive to the voltage across said capacitor includes:

an electromagnetic relay having a relay switch controlling said electric means;

a branch circuit connected in parallel with said capacitor and comprising the coil of said relay, a third rectifier and a normally non-conductive gaseous discharge device; said discharge device becoming conductive when the voltage across said capacitor reaches said predetermined voltage; said rectifier being poled to prevent current flow through said relay coil when the voltage across said capacitor is of said first polarity.

3. The combination according to claim 2 in which said electric means is a power relay having coil means controlled by said relay switch.

4. In a control system for a clothes drying machine having an electric motor for rotating a drum in order to tumble clothes contained therein to be dried, heating means arranged to heat clothes within said drum, and spaced electrodes positioned within said drum and exposed to clothes therein in order to be electrically bridged by wet clothes tumbled in said drum; the combination therewith of:

an alternating current supply circuit including at least first and second supply lines;

a first electromagnetic power relay having a first coil and a normally open first relay switch closed upon energization of said first coil, said first relay being efiective when energized to connect said heating means to said supply circuit;

a second electromagnetic relay having a second coil and a normally closed second relay switch opened upon energization of said second coil;

a momentary start switch having normally open contacts for initiating operation of the dryer; I

first conductor means connecting said first relay switch and said start switch in parallel between said first supply line and a junction;

second conductor means connecting said first relay coil and said second relay switch in series between said junction and said second supply line;

a pair of terminal leads coupled to said junction and said second supply line for energization with alternating voltage;

a capacitor connected at one side to said first terminal lead; a

a first rectifier circuit connected between the second side of capacitor and the second terminal lead to convert said alternating voltage into an unidirectional voltage of a first polarity across said capacitor, said first rectifier circuit consisting of said electrodes, a first resistor and a first rectifier connected in series;

a second rectifier circuit connected in parallel with said Q u first rectifier circuit and including in series connection a second resistor and a second rectifier oppositely poled with respect to said first rectifier to convert said alternating voltage into a unidirectional voltage across said capacitor of a second polarity opposite to that of said first unidirectional voltage;

said first resistor having a resistance value less than that of said second resistor so that said first rectifier circuit is effective to charge said capacitor with a voltage of said first polarity when said electrodes are bridged by damp clothes tumbled in said drum;

said second rectifier circuit with said capacitor comprising an R-C timing circuit effective to discharge and then recharge said capacitor to a predetermined voltage of said second polarity when the electrical resistance of clothes engaging said electrodes is greater than a predetermined value;

a branch circuit connected in parallel with said capacitor and comprising in series connection said second relay coil, a third rectifier and a normally non-conductive gaseous discharge device, said discharge device becoming conductive when the voltage across said capacitor reaches said predetermined value; said third rectifier being poled to prevent current flow through said second relay coil when the voltage across said capacitor is of said first polarity.

5. The combination according to claim 4 including a second branch circuit connected in parallel with said electrodes and said first resistor; said branch circuit including in series connection second normally open momentary contacts of said start switch and a third resistor having a resistance value less than that of said second resistor for charging said capacitor with a voltage of said first polarity when said start switch is actuated.

6. The combination according to claim 4 wherein said second resistor is a variable resistance potentiometer having a movable contact connected to shunt selected portions of said second resist-or for determining the time interval in which the voltage across said capacitor reaches said predetermined value.

7. The combination according to claim 4 including a thermostatic cool-down switch opened at low temperatures and closed while said clothes are being heated by said heater means; a third normally open relay switch closed upon energization of said first relay coil; a first energizing circuit for said motor including said third relay switch connecting said motor to said supply circuit; and a second energizing circuit including said cool-down switch when closed connecting said motor to said supply circuit independently of said third relay switch for continued operation of said motor during a cool-down period following deenergization of said heating means.

8. The combination according to claim 4 including a door switch operated by a door providing access to the interior of said dryer drum and having contacts opened in response to opening of said door; said door switch contacts being connected in said second supply line to effect the deenergization of said first relay coil and said terminal leads when said door is opened.

9. The combination according to claim 8 wherein said door switch has additional contacts closed in response to opening of said door; a discharge circuit comprising a discharge resistor in series with said additional contacts connected across said capacitor to discharge the latter in response to opening of said door.

References Cited UNITED STATES PATENTS 3,229,379 1/1966 Heidtmann 3445 FREDERICK L. MATTESON, JR., Primary Examiner.

J. J. CAMBY, Assistant Examiner. 

1. IN A CONTROL SYSTEM FOR A CLOTHES DRYING MACHINE HAVING AN ELECTRIC MOTOR FOR ROTATING A DRUM IN ORDER TO TUMBLE CLOTHES CONTAINED THEREIN TO BE DRIED, HEATING MEANS ARRANGED TO HEAT CLOTHES WITHIN SAID DRUM, AND SPACED ELECTRODES POSITIONED WITHIN SAID DRUM AND EXPOSED TO CLOTHES THEREIN IN ORDER TO BE ELECTRICALLY BRIDGE BY WET CLOTHES TUMBLED IN SAID DRUM: THE COMBINATION THEREWITH OF: AN ALTERNATING CURRENT SUPPLY CIRCUIT; A PAIR OF TERMINAL LEADS COUPLED TO SAID SUPPLY CIRCUIT FOR ENERGIZATIN WITH AN ALTERNATING VOLTAGE; A CAPACITOR; A FIRST RECTIFIER CIRCUIT INCLUDING IN SERIES WITH SAID ELECTRODES A FIRST RESISTOR AND A FIRST RECTIFIER; SAID FIRST RECTIFIER CIRCUIT BEING CONNECTED IN SERIES WITH SAID CAPACITOR ACROSS SAID TERMINAL LEADS TO CONVERT SAID ALTERNATING VOLTAGE INTO A UNIDIRECTINAL VOLTAGE OF A FIRST POLARITY ACROSS SAID CAPACITOR; A SECOND RECTIFIER CIRCUIT CONNECTED IN PARALLEL WITH SAID FIRST RECTIFIER CIRCUIT AND INCLUDING IN SERIES CONNECTION A SECOND RESISTOR AND A SECOND RECTIFIER OPPOSITELY POLED WITH RESPECT TO SAID FIRST RECTIFIER TO CONVERT SAID ALTERNATING VOLTAGE INTO AN UNIDIRECTIONAL VOLTAGE ACROSS SAID CAPACITOR OF A SECOND POLARITY OPPOSITE TO THAT SAID FIRST UNIDIRECTIONAL VOLTAGE; SAID FIRST RESISTOR HAVING A RESISTANCE VALUE LESS THAN THAT OF SAID SECOND RESISTOR SO THAT SAID FIRST RECTIFIER CIRCUIT IS EFFECTIVE TO CHARGE SAID CAPACITOR WITH VOLTAGE OF SAID FIRST POLARITY WHEN SAID ELECTRODES ARE BRIDGED BY DAMP CLOTHES TUMBLED IN SAID DRUM; SAID SECOND RECTIFIER CIRCUIT EFFECTIVE TO DISCHARGE AND ING AN R-C TIMING CIRCUIT EFFECTIVE TO DISCHARGE AND THEN RECHARGE SAID CAPACITOR TO A PREDETERMINED VOLTAGE OF SAID SECOND POLARITY WHEN THE ELECTRICAL RESISTANCE OF CLOTHES ENGAGING SAID ELECTRODES IS GREATER THAN A PREDETERMINED VALUE; ELECTRIC MEANS EFFECTIVE WHEN ENERGIZED TO CONNECT SAID HEATING MEANS TO SAID SUPPLY CIRCUIT; AND MEANS RESPONSIVE TO THE VOLTAGE ACROSS SAID CAPACITOR TO DEENERGIZED SAID ELECTRIC MEANS WHEN SAID CAPACITOR CHARGES TO SAID PREDETERMINED VOLTAGE OF SAID SECOND POLARITY. 